Method of determining ore concentration



April 30, 1968 J. L. DEWEY 3,380,897

METHOD OF DETERMINING ORE CONCENTRATION Filed Nov. 16, 1964 2 Sheets-Sheet 1 INVENTOR.

JOHN L. DEWEY April 30, 1968 Filed Nov.

CELL EMF VOLTS J. DEWEY 3,380,897

METHOD OF DETERMINING ORE CONCENTRATION 2 Sheets-Sheet 2 IO 20 3O 4O 5O 6O 7O CELL CURRENT- THOUSANDS OF AMPERES ALUMINA INVENTOR.

JOHN L. DEWEY United States Patent Ofice Fatented Apr. 30, IQGS 3,380,897 METHOD OF DETERMENHNG GEE CONCENTRATHON John L. Dewey, Florence, Alan, assignor to Reynolds Metals Company, Richmond, Va, a corporation of Delaware Filed Nov. 16, 1964, Ser. No. 413,463 Claims. (Cl. 204-1) This invention relates to a method of determining the concentration of an ore in a molten electrolyte. More specifically, although not limited thereto, this invention relates to a method for determining the alumina concentration in a conventional alumina reduction cell.

The production of aluminum by electrolysis of alumina, A1 0 dissolved in a molten salt electrolyte, e.g., cryolite, is an old and well known process. For a history of the aluminum industry reference is made to an excellent monograph by T. G. Pearson, The Chemical Background .of the Aluminum Industry, published by The Royal Institute of Chemistry, London, England, as reprinted with minor corrections in September 1957. The alumina is broken down into its components, the aluminum collecting in a pool of molten metal at the cathode and the oxygen being liberated on the carbon anode with which it combines chemically to form gaseous carbon dioxide and carbon monoxide. Two types of cells distinguished primarily by their anodes are in common use, viz., a prebake cell using an anode comprising a number of relatively small carbon blocks that are prebaked before being installed in the cell, and a Soderberg cell using an anode comprising a single, large mass of carbon that is baked in situ during operation of the cell. Electrolyte comprising a solution of aluminum oxide in molten cryolite is disposed between the carbon anode and the molten aluminum pool, and is covered by a crust of frozen electrolyte and alumina to diminish heat losses from the cell. During operation of the cell portions of the crust are broken in periodically to replenish the alumina in the electrolyte, and metal is siphoned periodically from the molten aluminum pool.

The principal object of this invention is to provide a method of determining the concentration of ore dissolved in the electrolyte of a reduction cell.

Another object is to provide a rapid method of determining the concentration of alumina dissolved in the electrolyte .of alumina reduction cells by using the electrical characteristics of the operating cell.

Yet another object is to provide a method of analysis of ore concentration dissolved in the electrolyte of a reduction cell suitable for programming high-speed computers, whereby the cell operator will be provided with an ore analysis of each cell substantially instantaneously at his request.

Further objects and advantages of my invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 shows schematically several cells in an alumina reduction potline with elements used in the practice of the method of this invention;

FIG. 2 illustrates a graphical method of determining the zero-current intercept of a tangent to the voltagecurrent curve of a reduction cell; and

FIG. 3 shows the relationship between the zero-current intercept, E and the electrolyte alumina content found in a particular design of an alumina reduction cell.

One of the important control parameters of cell operation is the dissolved alumina content of the electrolyte. If the alumina content falls too low the cell will have an anode effect wherein the voltage across the cell suddenly increases by to volts with attendant wastage of electric power and undesirable heating of the cell. If

the cell is fed While the alumina content is too high, substantial portions of the alumina will settle on the bottom of the cell, giving rise to an upset, or sick condition, during which the production rate is sharply decreased.

The need for a method of rapidly determining the alumina content has long been recognized and various methods have been proposed, but the method still commonly in use involves taking a sample of the electrolyte from the pot, carrying it to the analytical laboratory, and analyzing it by one or" the several time-consuming chemical procedures. Since the elapsed time may be upwards of two hours, the cell operator, who needs the information immediately, is forced to rely on his judgment and skill to decide when, or if, the cell should be fed, and how much it should be fed. Operators at times have even adopted the practice of feeding the cell only at anode effects because they realized that their judgment was at best imperfect and they felt that anode effects were preferable to sick pots.

During a series of investigations of the effects of alumina content and anode-to-cathode spacing on the behavior of alumina reduction cells it was discovered that the value of the zero-current intercept of the tangent to the voltage-current curve varied substantially inversely with the dissolved alumina content of the electrolyte. This discovery was surprising in that the intercept, variously referred to as back EMF. and decomposition potential in the literature, has been considered to be substantially constant, and has been so used in several electromechanical devices for computing the electrical resistance of cell lines comprising a plurality of alumina reduction cells electrically connected in series.

This discovery led to the conclusion that determination of the zero-current intercept would be a desirable step in a method of determining the alumina content or" alurnina reduction cells. It was further recognized that the method would be especially adaptable to the use of high speed, electronic computers which could be programed to select the voltage and current data from a cell line of perhaps cells, compute the zero-current intercepts of the tangents to the voltage-current curve, and obtain therefrom the corresponding alumina concentrations. The information may be displayed to the operators within a short time, thereby eliminating the need for the timeconsuming chemical analyses and the need for the operators judgment and skill in performing his feeding operations.

Turning now to FIG. 1, several alumina reduction cells 14 are connected in series with a source of unidirectional electric power (not shown) by a current conducting bus 23 having a current shunt 21. A conventional ammeter 22 is connected to the current shunt 21 to indicate the value of the current in amperes flowing in the bus 23. Each cell 24 comprises an anode 11 connected to the bus 23 and suspended into the electrolyte 14, a cathode 12 which serves as an electrical connection between the bus 23 and the molten aluminum pool 13, a layer of electrolyte 14- comprising a solution of alumina in molten cryolite, and a crust 15 of a mixture of frozen electrolyte and alumina. A voltmeter 20 is connected through electrical conductors 18 and 19 to the bus 23 at points 16 and 17 and hence across each cell 24 for displaying the potential drop across each cell.

Curve A in FIG. 2 illustrates the determination of the zero-current intercept, E at a time when the percentage of alumina dissolved in the electrolyte is relatively low. A first current-voltage point (a) is obtained by noting substantially simultaneously the respective values of the cell voltage B and the line current I and plotting E against 1,, on rectangular coordinate graph paper. Soon thereafter the line current is changed to another value,

, I and after waiting several seconds for the cell to adjust to the new current, a voltage E corresponding to the line current 1,, is noted. The point (b) is then located on the graph. With a straight-edge a line is drawn through the points (a) and (b) and extended to the voltage axis of the graph. The curve A at the voltage axis is designated the zero-current intercept, E

FIG. 3 depicts the relationship between E and the percentage of dissolved alumina in the electrolyte for a particular design of alumina reduction cell, in this case, a prebake cell using carbon blocks which, when new, were about 20 inches long by 18 inches wide. The calibration curve of FIG. 3 or some other suitable function of zero-current intercept values is obtained for a given cell design by making a series of determinations of E as the alumina content of the cell is allowed to vary, and determining for each value of E, a corresponding value of electrolyte alumina content by chemical analysis of samples of electrolyte taken from the cell at the appropriate times. Once the function of zero-current intercept values such as the calibration curve is obtained for the given cell design it may be used thereafter to relate values of E; obtained on other alumina reduction cells of the same design to the percentage of dissolved alumina in the electrolyte of said other cells at the time said values of E were determined. The alumina content corresponding to the value of E; determined by curve A of FIG. 2 is obtained from FIG. 3 by the steps of:

(l) Locating the value of E, on the E; axis of FIG. 3;

(2) Proceeding parallel to the alumina content axis to the intersection with the calibration curve;

(3) Proceeding from said intersection perpendicularly to said alumina content axis; and

(4) Reading the alumina content at the intersection of said perpendicular line with said alumina content axis.

With reference to FIG. 2 it may be deduced that any error in the value of E, will be a direct function of the errors in determining the values of cell voltage and line current for the points (a) and (b), and an inverse function of the difference between I and l Experience indicates that, when using voltage and current sensing instruments accurate to 0.1% of full scale and a current difierence of about 15% of normal line load, the dissolved alumina content values differ from the analytically determined values by less than 1% alumina. The accuracy of the determination can, of course, be improved by making duplicate determinations of the points (a) and (b) of FIG. 2.

Since the method of this invention determines primarily the dissolved alumina content of the electrolyte, determinations obtained immediately after heavy feedings of alumina to the cell may be lower than chemical analyses because fine alumina particles can become suspended in the electrolyte after such feedings and require thirty minutes or more to settle out.

When the alumina reduction cell is in the sick condition because of an accumulation of sludge on the bottom of the cell, or, in the case of the multiple block anode cells where one of the blocks is carrying an excessively high current, the cell votage may comprise a low frequency alternating component with an amplitude at times exceeding 0.1 volt. The presence of this alternating component acts to increase the error of the values determined with the method of this invention.

The values of dissolved alumina obtained with the method of the invention are substantially independent of the anode-to-cathode spacing of the cell, the electrolyte temperature, and the various ohmic resistances such as the resistance in the anode assembly, the cathode assembly, and between the molten aluminum layer and the cathode provided that these parameters do not change in value during the determination of a pair of points (a) and (b).

The voltage-current traces of some cell designs may be slightly curved over the range of current values most l convenient for use in determining the points (a) and (b) of FIG. 2. For these designs it is preferable to preselect relatively narrow ranges of currents within which I and 1,, may be conveniently and respectively placed, and thereafter to use only those points (a) and (b) for which I and 1,, are within their respective ranges, thereby effectively limiting the analytical errors to the cooperative effects of instrument quality and the current difference between the values of I and I While for purposes of description I have described a specific method, it will be apparent that changes and modifications can be made therein by those skilled in the art upon being apprised of the discovery of this invention without departing from the spirit of my invention or the scope of the appended claims.

What is claimed is: 1. A method of determining an unknown concentration of an electrochemically-active material in the bath of an electrolytic reduction cell comprising the steps of:

ascertaining the values of a plurality of projected zerocurrent intercepts for a corresponding number of voltage-current curves for operation of the cell at different bath concentrations, and recording the various projected zero-current intercept values corresponding to each of the different bath concentrations;

ascertaining the projected zero-current intercept value corresponding to the unknown bath concentration that is to be determined;

comparing the projected zero-current intercept value corresponding to said unknown bath concentration with a function of said recorded zero-current intercept values; and

determining the unknown bath concentration as being substantially the same as that hath concentration associated with said function at the intercept value corresponding to the unknown hath concentration, whereby the unknown bath concentration is determined without performing a separate chemical analysis therefor.

2. A method of determining an unknown bath alumina concentration in an electrolytic reduction cell for the production of aluminum comprising the steps of:

ascertaining the values of a plurality of projected zerocurrent intercepts of the tangents to a corresponding number of voltage-current curves for operation of the cell at difierent bath alumina concentrations, and recording the various projected zero-current intercept values corresponding to each of the different bath alumina concentrations;

ascertaining the projected zero-current intercept value corresponding to the unknown bath alumina concentration that is to be determined;

comparing the projected zero-current intercept value corresponding to said unknown bath alumina concentration with a function of said recorded zerocurrent intercepts; and

determining the unknown bath alumina con-centration as being substantially the same as that bath alumina concentration associated with said function for the intercept value equal to said projected zero-current intercept value corresponding to the unknown bath alumina concentration, whereby the unknown bath alumina concentration is determined without performing a separate chemical analysis therefor.

3. The method of claim 2 wherein the projected zerocurrent values are determined by a method comprising the steps of:

measuring the current and voltage of said cell while said bath is operating at a first operating point; changing the current through said cell so that said bath operates at a second operating point;

measuring the current and voltage of said cell at said second operating point;

recording the measured currents and voltages of said first and second operating points in accordance with a system of voltage-current coordinates; and extrapolating from the thusly recorded first and second points to determine the zero-current intercept value. 4. The method of claim 3 wherein the current and voltage values for said first and second operating points are recorded on a graph having rectangular coordinates; and

said zero-current intercept value is determined by drawing a line through the thusly plotted points to intersect the voltage axis at the zero-current intercept. 5. The method of determining the concentration of alumina in the electrolyte of an alumina reduction cell, comprising the steps of establishing two points on the voltage-current curve of the cell, by measuring the current and voltage at one cell operating point, changing the current through the cell, and measuring the changed current and corresponding cell voltage; plotting these points on a graph, drawing a line through said points to intersect the voltage axis and determine the zero-current intercept; repeating the steps of determining the zero-current intercept for various concentrations of alumina, thereby ascertaining the alumina concentration for each zero-current intercept; plotting a curve of the values of zero-current intercepts versus alumina concentrations; and subsequently determining unknown alumina concentration by ascertaining the zero-current intercept for the unknown concentration and relating it to said curve.

References Cited UNITED STATES PATENTS 3,034,972 5/1962 Lewis 20467 3,141,835 7/1964 Rolin et a1 204-1.1 3,294,656 12/1966 Schrnitt 204-67 3,324,013 6/1967 Dewing 2041.1

HOWARD S. WILLIAMS, Primary Examiner.

JOHN E. MACK, Examiner.

T. TUNG, Assistant Examiner. 

1. A METHOD OF DETERMINING AN UNKNOWN CONCENTRATION OF AN ELECTROCHEMICALLY-ACTIVE MATERIAL IN THE BATH OF AN ELECTROLYTIC REDUCTION CELL COMPRISING THE STEPS OF: ASCERTAINING THE VALUES OF A PLURALITY OF PROJECTED ZEROCURRENT INTERCEPTS FOR A CORRESPONDING NUMBER OF VOLTAGE-CURRENT CURVES FOR OPERATION OF THE CELL AT DIFFERENT BATH CONCENTRATIONS, AND RECORDING THE VARIOUS PROJECTED ZERO-CURRENT INTERCEPT VALUES CORRESPONDING TO EACH OF THE DIFFERENT BATH CONCENTRATIONSN; ASCERTAINING THE PROJECTED ZERO-CURRENT INTERCEPT VALUE CORRESPONDING TO THE UNKNOWN BATH CONCENTRATION THAT IS TO BE DETERMINED; COMPARING THE PROJECTED ZERO-CURRENT INTERCEPT VALUE CORRESPONDING TO SAID UNKNOWN BATH CONCENTRATION WITH A FUNCTION OF SAID RECORDED ZERO-CURRENT INTERCEPT VALUES; AND 